This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
The formula for determining the strength of wooden columns having flat or square ends was deduced from exhaustive tests of full-size specimens, made at the Water-town Arsenal, Mass., and may be expressed as follows:
S=U-(Ul/100d), in which S = ultimate strength of column per square inch of section; U= ultimate compressive strength of the material per square inch; I = length of the column in inches; d = dimension of the least side of the column in inches. The above formula may be applied to all wooden columns, the length or height of which is not under 10 times nor over
45 times the dimension of the least side. In other words, l / d should not be less than 10 nor more than 45. If the length is less than 10 times the least side, the direct compressive strength of the material per square inch, multiplied by the sectional area of the column in square inches, will give the strength of the column. If the length is over 45 times the least side, this formula will not apply; in such cases, provision must be made for bracing the column in all directions, or the load upon it must be greatly reduced. Columns of this dimension, however, seldom occur in building practice.
Having determined S, the breaking load per square inch of section, the safe load per square inch will be obtained by dividing by the required factor of safety, 4, 5, or 6. This result, multiplied by the sectional area, will give the safe load the column will sustain.
What safe load will a 10" X 12" Northern yellow-pine column 20 ft. long support, provided a factor of safety of 5 used?
Solution - In the formula S = U=(Ul/100d), the value of U for Northern yellow pine is given in Table IX, page 71, as 4,000; I is equal to 20 ft. X 12 in. = 240 in.; and the least side of the column, or d, is 10 in. By substituting these values in the formula, S = 4,000-[(4000x240)/(100x10)]=3,040 lb. the ultimate or breaking strength of the column per square inch of cross-section.
Then, 3,040 / 5 (factor of safety) = 608 lb., the safe strength of the column to compression per square inch of section.
The sectional area of the column is 12 in. X 10 in. = 120 sq. in.; hence, the safe load that the column will support is equal to 608 X 120, or 72,960 lb.
For slow-burning construction, the Fire Underwriters' Association will not allow the use of square wooden columns less than 8 in. on a side; so, although the actual required size of column to safely sustain the load might be much less than 8 in., it is not advisable in first-class buildings of this construction to use wooden columns less than this size.
Large timber posts - that is, posts not under 8 or 10 in. least dimension - are considered as offering more resistance to Are than cast-iron columns; hence, they are often used, especially in mill construction, in preference to the latter.
Care should be taken in selecting timber for columns or posts to obtain only seasoned wood, without wind or twist. free from defects likely to affect its strength. The ends of the posts should be cut square, so as to take a uniform bearing at the base and cap plates. The timber commonly used forposts is yellow pine, which lncludes the Northern, Southern, or Georgia pines; white pine, spruce, and Oregon spruce are frequently used, and in some instances, oak.
Details of the usual cast-iron caps and bases which are used with wooden columns are given on page 197.